High molecular weight hydroxyl group-containing polyurethanes soluble in organic solvents have long been of commercial significance, especially in the production of two-component coating compositions and adhesives (cf. "Bayer-Kunststoffe", 3rd Edition 1963, pages 132 et seq and pages 422 et seq).
The production of hydroxyl polyurethanes of this type is described, for example, in German Pat. No. 962,522 and 1,012,456, and is carried out by adding a diisocyanate, for example, tolylene diisocyanate, p-phenylene diisocyanate, diphenyl methane-4,4'-diisocyanate or hexamethylene diisocyanate, at a temperature of from 80.degree. to 90.degree. C. to linear compounds having two hydroxyl groups and a molecular weight below 5000, for example, polyesters of succinic acid, adipic acid, sebacic acid, dodecane dicarboxylic acid or phthalic acid, and ethylene glycol, polyethylene glycols, propylene glycol, polypropylene glycols, butane diol, hexane diol or neopentyl glycol, or polyethers, for example, based on ethylene oxide, propylene oxide and butylene oxide, the NCO:OH ratio being .ltoreq. 1 and preferably from 0.92 to 0.99, briefly stirring the reaction mixture in order to homogenize it and heating the thus-homogenized mixture in a reaction vessel for about 10 hours at 120.degree. C. in order to complete the reaction. The end products of this process are soluble, high molecular weight rubber-like products having an average molecular weight (weight average) of from about 30,000 to 250,000, preferably from about 50,000 to 150,000, depending upon the NCO:OH ratio.
The increasing significance of high molecular weight hydroxyl group-containing polyuretnanes soluble in organic solvents as starting components for the production of high-quality plastics, for example, for the adhesives sector, for coating textiles, for lamination purposes, for dressing leather, etc. is resulting in continuously increasing quality requirements in regard to the polyurethane components, more especially in regard to their consistency, reproducibility and the specification range of their property spectrum.
However, it has long been known that the reaction of isocyanates with compounds containing reactive hydrogen atoms carried out in the melt in the manner described above is inconsistent, difficult to control and equally difficult to reproduce (cf. German Pat. No. 962,552 and Otto Bayer: "Das Diisocyanat-Polyadditions-Verfahren" in Kunststoff-Handbuch, Vol. VII, page 20, published by Richard Vieweg and August Hochtlen, Carl-Hanser-Verlag, Munich, 1966 and Houben-Weyl, Vol. 14, part 2, pages 72 to 73). According to the above-mentioned literature references, the reproducibility of the polyurethane reaction is promoted by using inert organic solvents, more especially hydrocarbons, such as benzene, toluene, xylene, chlorobenzene and o-dichlorobenzene, the reaction of 1,4-butane diol with hexamethylene diisocyanate in chlorobenzene to form an injection-moldable polyurethane powder having an average molecular weight of up to 15,000 and melting point of 184.degree. C. being quoted as an Example.
There has also been no shortage of attempts to react high molecular weight polyols, such as polyesters or polyethers, with diisocyanates in this way. However, considerable difficulties were involved in obtaining substantially the same molecular weights as in the melt process. According to U.S. Pat. No. 2,223,672 for example, the reaction of equimolar quantities of a linear aliphatic polyester containing hydroxyl groups (molecular weight from 1500 to 3000) with an aromatic diisocyanate in boiling aromatic solvents, such as benzene, toluene or xylene, gives polyurethanes having an average molecular weight of from 10,000 to 15,000, corresponding to a solution viscosity of from 400 to 3000 centipoises at room temperature for the solutions adjusted to a polyurethane content of from 10 to 30%, by weight, with a polar solvent, preferably methyl ethyl ketone. Although solutions of this type may be used, for example, as laminating adhesives, they are not suitable for the production of high-quality textile coatings both on account of their low molecular weight and on account of their low solution viscosity.
German Auslegeschrift No. 1,301,124 describes the production of polyurethanes by reacting a mixture of (a) 1 mol of a polyester (molecular weight from 1200 to 5000) containing terminal hydroxyl groups, (b) from 2 to 4 mols of a diol containing primary hydroxyl groups and, optionally, (c) at most 0.5 mol of a triol containing primary and/or secondary hydroxyl groups, with aliphatic diisocyanates in an NCO:OH ratio of .ltoreq. 1 in the presence of chlorinated aromatic solvents with a boiling point of from 120.degree. to 200.degree. C. which are inert to isocyanate groups, for example, chlorobenzene or o-dichlorobenzene. However, the polyurethanes obtained are insoluble in most organic solvents, such as methylene chloride, chloroform, trichloroethylene, cyclohexane, benzene, toluene, acetone, methyl ethyl ketone, ethers, dioxane, tetrahydrofuran, pyridine, dimethyl formamide and dimethyl sulphoxide, and may only be thermoplastically processed following removal of the chlorinated aromatic solvent.
The reaction of linear dihydroxyl compounds with diisocyanates in an NCO:OH ratio of .ltoreq. 1, preferably from about 0.95 to 1.0, in the absence of chain extenders to form very high molecular weight (average molecular weights up to more than 250,000) hydroxyl group-containing polyurethanes, which are soluble in most organic solvents, in the presence of solvents, was described for the first time in DOS No. 2,149,836 in the form of a three-stage process. This three-stage process is distinguished in particular by the fact that (a) in a first reaction stage the starting components are reacted at a "specific reaction temperature" adjusted to an accuracy of .+-. 1.degree. C. in the range of from 100.degree. to 160.degree. C., preferably from 115.degree. to 140.degree. C., in the presence of non-polar or weakly polar solvents with continuous monitoring of viscosity until a maximum viscosity is reached; (b) in a second reaction stage the reaction mixture is left to react for from 12 to 72 hours at from 60.degree. to 100.degree. and preferably at from 70.degree. to 90.degree. C. until no more free isocyanate may be detected; and (c) in a third reaction stage the solvent is removed in known manner at reduced pressure and elevated temperature.
Suitable solvents are non-polar or weakly polar solvents having an E.sub.T -value of less than 35 (for the definition of the E.sub.T -value as a measure of solvent polarity, cf. Fortschritt chem. Forsch, vol 11/1, pages 1 to 73), for example, toluene (E.sub.T = 33.9), xylene (E.sub.T = 33.2) or cyclohexane (E.sub.T = 34.0), di-n-butyl ether (E.sub.T = 33.4), carbon disulphide (E.sub.T = 32.6), carbon tetrachloride (E.sub.T = 32.5), n-hexane (ligroin) (E.sub.T = 30.9); toluene is preferred. The reaction is carried out using from 30 to 80, preferably from 50 to 60, parts, by weight of solvent to from 70 to 20, preferably from 50 to 40, parts, by weight, of the starting components, the solvent having effectively to dissolve the starting components and to dissolve the polyaddition product formed at least to such an extent that a homogeneous phase is always present under the reaction conditions.
The "specific reaction temperature" is a parameter which has to be determined in a preliminary test for each diol-diisocyanate combination. It is generally from 115 to 140.degree. C. In the context of the invention, the "specific reaction temperature" is that temperature at which the highest viscosity (i.e. the highest molecular weight or the lowest proportion of allophanate branchings) of the fully reacted polyurethane solution is obtained with a given NCO:OH ratio of the reaction mixture (cf. FIGS. 1 to 3). The "specific reaction temperature" may readily be determined by keeping several portions of the same reaction mixture (with the same quantities of solvent in each case) at different temperatures in the range of from 100.degree. to 160.degree. C. until all the NCO-groups have disappeared, and subsequently determining the viscosity of the polymer solution formed at room temperature. Since the "specific reaction temperatures" are generally above the boiling point of the solvent used, the reactions are carried out in reactors designed for corresponding excess pressures.
This process, which is described in DOS No. 2,149,836, is highly reproducible in terms of large scale production and leads to chemically particularly consistent linear polyurethanes which are completely soluble in most organic solvents without any signs of gelling or swelling and which, in combination with polyisocyanates, are eminently suitable for the production of high-quality two-component testile coatings and for the production of polyurethane adhesives and leather dressings.
Unfortunately, the process described in DOS No. 2,149,836, by comparison with conventional melt polyaddition, requires a much longer reaction time, so that in some cases the improvement in the quality of the end products is prohibitively offset by the poorer volume-time yield of the production unit. In addition, the scope of application of the process is limited to an extent by the necessary solubility of the end polyurethane in the only weakly polar reaction medium. In particular, polyurethanes of the type into which additional urethane segments are incorporated by using short-chain diols having molecular weights of from about 60 to 300 in order to obtain particular properties and which are readily soluble in the most commonly used solvents, such as ethyl acetate, acetone, methyl ethyl ketone or methyl glycol acetate, show a marked thixotropic effect in the weakly polar solvents used as reaction medium, for example, in toluene. This not only makes it very difficult to assess the degree of polyaddition by continuous monitoring of the viscosity of the reaction medium, but it may also lead to interruptions in production on account of the inadequate stirrability and pumpability of the reaction solution.